To reduce the complexity of packaging integrated circuit chips, techniques have been developed for directly mounting the chip on the board. This so-called direct chip attach (DCA) or chip-on-board (COB) packaging can provide the smallest and cheapest package among various packaging technologies.
The devices directly attached on the boards are usually electrically connected by wire bond or tape automated bonding (TAB) technology which typically requires protection from process chemicals, dust and humidity. Thus, a resin encapsulant is placed over the chip and the wiring connecting the chip pads with the board.
In case of a malfunction of such an encapsulated chip, the chip generally cannot be removed and replaced without damaging the circuitry and the organic carrier (board). Replacing a defective chip with a new one using the normal direct chip attach soldering process is also not possible. Thus, a fully assembled board generally often has to be scrapped when the presence of a defective chip is detected.
In the IBM Technical Disclosure Bulletin, Vol. 29, No. 3, August 1986, p. 1433, a reworkable chip-on-board package is disclosed, in which chips are back bonded in a recess. In case of malfunction, a second chip is back bonded on top of the defective chip. Before the replacement chip is fixed on top of the first chip, the wire bond pads are removed. This method requires a recess for the chip and allows only one rework cycle per chip.
IBM Technical Disclosure Bulletin, Vol. 31, No. 3, August 1988, p. 39, shows the test and repair of direct chip attach modules which are C4-bonded (controlled collapsed chip connection), in which a thermode is placed on the back side of the substrate on which the chip is directly attached. The thermode is used to reflow the solder, thereby allowing removal of the chip. This method can only be used with single layer substrates and nonsealed chips.
The present invention as claimed therefore is intended to remedy these drawbacks by providing a new replacement technique for directly attached chips.